The purpose of the Flow Cytometry Shared Resource (FCSR) is to provide comprehensive cytoflourometric analysis and sorting to the YCC investigators and the broader YSM community. The facility not only provides, maintains, and operates the instruments, but also trains users, develops techniques, provides protocols, and manages access and the financial aspects of the Shared Resource. Until the winter of 2004, YSM had two entities that provided FACS services: the YCC FCSR and the Immunobiology (1MB) FACS facility. The former was supported by the YCC grant and user fees while the latter was subsidized in part by Howard Hughes Medical Institute (HHMI), which owned some of the instruments and paid their service contracts. The YCC Shared Resource had a slow-speed sorter and also had access to a FACSCalibur. In winter of 2002, YSM opened a new research building which was to house all of 1MB as well as many YSM faculty members doing immunologically-related research. Using space donated in part by the Dept. of Medicine and in part by Immunobiology (1MB), the school agreed to customdesign space to house an expanded FACS Core that would now be open to 1MB as well as the Dept. of Medicine. In addition, individual units within Medicine donated three FACS Calibur machines to the newly constituted core. At the same time, the FACS Core Director (now YCC FCSR Director) had obtained a shared instrument grant for a new sorter, and a FACS Aria was installed in the fall of 2002. Finally, the YSM purchased for this facility a used MoFlo sorter. Since the YCC FCSR continued to have limited equipment and space, it was decided to improve the quality and scope of services available to YCC members by merging the two facilities. In concert with this, the YCC agreed to partially subsidize the purchase of an LSRII analyzer and to direct its current user subsidy budget towards an employee of the now-merged facility. The facility thus has three high-speed sorters, 4 FACSCaliburs, a FACScan and LSRII. These instruments are housed in TAG S613 and S617, with a Mac and a PC based workstation for data analysis adjacent. It is used by 117 different Pis and has 486 trained users, who together used 3,952 hrs of sorting and 11,915 hrs of analysis in 2005, of which 61 YCC members represented 80% (77% peer-reviewed) and 87% (83% peerreviewed) respectively of total use. Members of 7 of 8 of YCC Research Programs utilized the FCSR.